1. Field of the Invention
This invention relates to a method of document self-authentication, and in particular, it relates to a method of document self-authentication that preserves critical content such as signatures on the document with high resolution in the authentication data.
2. Description of Related Art
Original digital documents, which may include text, graphics, images, etc., are often printed, and the printed hard copy are distributed, copied, etc., and then often scanned back into digital form. Authenticating a scanned digital document refers to determining whether the scanned document is an authentic copy of the original digital document, i.e., whether the document has been altered while it was in the hard copy form. Alteration may occur due to deliberate effort or accidental events. Authentication of a document in a closed-loop process refers to generating a printed document that carries authentication information on the document itself, and authenticating the scanned-back document using the authentication information extracted from the scanned document. Such a printed document is said to be self-authenticating because no information other than what is on the printed document is required to authenticate its content.
Methods have been proposed to generate self-authenticating documents using barcode, in particular, two-dimensional (2d) barcode. Specifically, such methods include processing the content of the document (bitmap image, text, graphics, etc.) and converting it into authentication data which is a representation of the document content, encoding the authentication data in a 2d barcode (the authentication barcode), and printing the barcode on the same recording medium as the printed document. This results in a self-authenticating document. To authenticate such a printed document, the document is scanned to obtain scanned data that represents the content of the document, e.g. a bitmap image, text extracted by using an optical character recognition (OCR) technology, etc. The authentication barcode is also scanned and the authentication data contained therein is extracted. The scanned data is then compared to the authentication data to determine if any part of the printed document has been altered, i.e. whether the document is authentic. Some authentication technologies are able to determine what is altered, some merely determine whether any alterations have occurred.
Methods of authenticating a document using 2d barcode are described in commonly owned U.S. patent application Ser. Nos. 12/050,701 and 12/050,718, both filed on Mar. 18, 2008.
To generate barcodes that encode the content of the document, different types of content in the document are often treated differently. Text content may be encoded as text strings. Image content, which tends to be large in data size, is typically down-sampled and compressed to reduce its size before it is coded in barcode. Thus, the authentication data stored in the barcode that represent image content typically has a lower resolution than the original image data in the document.
Within a document, there are often areas containing critical content, such as signatures. In conventional document authentication techniques, a signature is typically treated as an image, i.e., the signature image is down-samples and compressed and them coded in barcode. Thus, during the authentication step, the signature image extracted from the barcode on the scanned document often has poor quality, and the comparison of the extracted signature image with the signature on the scanned document can be unreliable.